1. Field of the Invention
The present invention relates to a method of producing aluminum fluoride. More particularly, it relates to a method of producing high purity aluminum fluoride with excellent efficiency from a gas containing a low concentration of a fluorine compound, for example, the off gas from an aluminum electrolytic cell.
2. Description of the Prior Art
Conventional industrial processes of producing aluminum fluoride can be roughly divided into the following two methods.
First, one widely commercialized method comprises absorbing a gas containing hydrogen fluoride at a relatively high conconcentration, e.g., the gas which results from the decomposition of a fluorite with sulfuric acid at elevated temperatures, into water, thereby obtaining a 15 to 25 % by weight aqueous hydrofluoric acid solution, and reacting the solution with alumina or hydrated alumina to obtain aluminum fluoride trihydrate. The concentration of hydrogen fluoride gas which results from the decomposition of a florite with sulfuric acid at elevated temperatures varies greatly. Generally, the fluorine concentration is around about 300 g/Nm.sup.3. This reaction is shown by Formulae (1) and (2). EQU HF(gas) + H.sub.2 0 .fwdarw. HF(soln) + H.sub.2 O (1) EQU 3HF(soln) + Al(OH).sub.3 .fwdarw. AlF.sub.3 .sup.. 3H.sub.2 O (2)
this reaction will be explained in detail hereinafter on the basis of these formulae, with reference to the case where alumina trihydrate, that is, aluminum hydroxide, is used as the alumina or hydrated alumina.
Further, the direct solid-gas contact reaction between hydrogen fluoride gas and alumina or hydrated alumina (dry method) is also included in this category.
The second method comprises absorbing an off-gas containing silicon tetrafluoride, e.g., which results from a phosphate rock treatment, into water, thereby obtaining about a 15% by weight aqueous hydrosilicofluoric acid solution, and then reacting the solution with alumina or hydrated alumina to obtain aluminum fluoride trihydrate. This reaction is as shown by Formulae (3) and (4). EQU 3SiF.sub.4 (gas) + 2H.sub.2 O .fwdarw. 2H.sub.2 SiF.sub.6 (soln) + SiO.sub.2 ( 3) EQU H.sub.2 SiF.sub.6 (soln) + 2Al(OH).sub.3 + 2H.sub.2 O .fwdarw. 2AlF.sub.3.sup.. 3H.sub.2 O + SiO.sub.2 ( 4)
the first method is disadvantageous from the economical point of view because gases of a high hydrogen fluoride concentration must be used as a starting material. On the other hand, the second method is economically advantageous in that off-gases from a phosphate rock treatment can be used as a starting material. Even in the latter method, however, the fluorine concentration of the gas must be higher than a certain level, e.g., about 2 to about 3 g/Nm.sup.3. When gases of a low fluorine concentration, such as the off gas from an aluminum electrolytic cell, are absorbed into water by the method described above, the aqueous solutions obtained have a hydrofluoric acid concentration of only 3 to 5 % by weight. Such solutions only yield a supersaturated solution of aluminum fluoride when reacted with alumina or hydrated alumina and produce no precipitates of aluminum fluoride trihydrate.
When such a supersaturated solution is heated to 80.degree. to 90.degree. C. and aluminum fluoride or the hydrate thereof is added as precipitation nuclei, the precipitation of aluminum fluoride trihydrate occurs at an extremely slow rate, leaving some aluminum fluoride still dissolved in the solution.
Consequently, this method is very unsatisfactory as an industrial method for the production of aluminum fluoride.
On the other hand, a precipitation method which comprises adding alumina as precipitation nuclei to a supersaturated solution of aluminum fluoride, thereby precipitating aluminum fluoride trihydrate, is disclosed in U.S. Pat. No. 3,533,924. According to this method, the aluminum fluoride trihydrate can be precipitated at high yield by a simple operation by reacting aluminum hydroxide with about a 3 to 5% by weight aqueous hydrofluoric acid solution which is obtained by washing the off gas from an aluminum electrolytic cell with water, thereby preparing a supersaturated solution of aluminum fluoride, and then by adding alumina to the resulting solution. However, in order to enhance the precipitation efficiency of aluminum fluoride trihydrate, alumina must be added in a large amount and almost all of the alumina is deposited as a co-precipitate comprising alumina and aluminum fluoride.
Consequently, a large size treating equipment is required and the product obtained contains an extremely high amount of alumina which is very difficult to remove, and, therefore, which limits usage of the product.
Other methods are known for the production of aluminum fluoride from gases having low concentrations of fluorine compounds. For example, the precipitation of hydrated aluminum fluoride can be effected by concentrating an aqueous solution having a low concentration of fluorine compounds which is obtained by a simple absorption of the gases by water and then adding alumina or hydrated alumina into the concentrated solution or by concentrating the aqueous solution having a low concentration of aluminum fluoride which is obtained by the addition of alumina or hydrated alumina to the aqueous solution having a low concentration of fluorine compounds.
These methods are, however, disadvantageous from the economical point of view because they require complicated concentration operations, and, therefore, entail high equipment costs.
As described above, following the prior art it is very difficult to produce high purity aluminum fluoride economically, efficiently and on an industrial scale, from gases of a low fluorine concentration.
Therefore, most of the fluorine compounds which are contained in such gases of low fluorine concentration or in solutions obtained by the absorption of such gases by water have been recovered as calcium fluoride or cryolite, whereby air pollution or waste water pollution has been prevented from occurring.
In the aqueous solutions obtained by a simple absorption of fluorine compound containing gases by water, that is, by a simple absorption which is not accompanied by reaction (for example, the formation of hydrofluoric acid by the absorption of hydrogen fluoride gas by water), the concentration of the fluorine compound is entirely fixed by the partial pressure equilibrium of the compound which is present between the gas phase and the liquid phase in the absorption system.
Therefore, when the gas absorbed has a low fluorine concentration, the aqueous solution obtained by a simple absorption of the gas by water has only a low fluorine concentration. Consequently, the addition of alumina or hydrated alumina to the thus obtained solution results in a supersaturated solution of aluminum fluoride from which high purity, solid aluminum fluoride cannot be obtained with a high efficiency.
In conventional methods, no attention has been given to the gas absorption step, and, therefore, no improvements in this step have been effected by the art.